The present invention relates to opaque, oriented polymeric films and to processes for making such films. More particularly, the invention relates to opaque, oriented polymeric films comprising a cavitated or voided core layer and at least one smooth outer or skin layer, and to processes for making the same.
In the packaging of and labeling of many food and non-food products, it is common practice to use an opaque polymeric film. Often, such opaque polymeric films are multi-layer films comprising a cavitated opaque, thermoplastic polymeric core layer having one or more solid skin layers thereon. The term xe2x80x9ccavitatedxe2x80x9d denotes the presence of numerous microscopic voids. The skin layers contribute various desirable characteristics to the packaging film, such as heat sealability, improved appearance, enhanced machine handling capabilities, and the like.
U.S. Pat. No. 4,632,869 describes a resin combination comprising a thermoplastic polymer matrix having dispersed therein as distinct phases, a multiplicity of small spherical solid particles of polybutylene terephthalate, the resin combination in opaque biaxially oriented polymeric film form, and the same oriented film structure having on at least one surface thereof a void-free thermoplastic skin layer. The process for preparing a biaxially oriented opaque film comprises providing said resin combination, forming an unoriented film of said resin combination and biaxially orienting said film to an extent sufficient to opacify the same. The process also provides for the opaque film having a void-free thermoplastic skin layer on at least one surface thereof. U.S. Pat. No. 4,632,869 is incorporated herein by reference in its entirety
U.S. Pat. No. 4,758,462 describes an opaque, biaxially oriented film structure which comprises: an expanded thermoplastic polymer matrix core layer within which is located a minor amount of a light-absorbing pigment and a strata of voids; positioned at least substantially within at least a substantial number of said voids is at least one void-initiating particle which is phase distinct and incompatible with the matrix material, the void space occupied by said particle being substantially less than the volume of said void, with one generally cross-sectional dimension of said particle, at least approximating a corresponding cross-sectional dimension of said void; the population of voids in said core and the thickness of said core being such as to cause a degree of opacity of less than 15% light transmission; and at least one void-free thermoplastic skin layer affixed to a surface of the core layer, said skin layer(s) being of a thickness such that the outer surfaces thereof do not, at least substantially, manifest the surface irregularities of said core layer. U.S. Pat. No. 4,758,462 is incorporated herein by reference in its entirety
U.S. Pat. No. 4,652,489 describes a sealable, opaque polyolefinic multilayer film composed of a polypropylene base layer, a non-sealable surface layer, and a sealable surface layer, and process therefor. The sealable surface layer has a low minimum sealing temperature and is made from a copolymer of propylene and ethylene or butene-1 units and/or a terpolymer of ethylene, propylene and butene-1 units. The non-sealable layer is a combination of a propylene homopolymer and a slip agent. The base layer contains an additive which is incompatible with polypropylene. The process includes stretching of the film, and during the stretching step, the polymer matrix is torn open around the additive particles to form vacuoles which give the base layer a degree of opacity. U.S. Pat. No. 4,652,489 is incorporated herein by reference in its entirety.
U.S. Pat. No. 6,183,856 describes a process for making an opaque, oriented polymeric film structure comprising preparing a melt containing a polymeric crystallizable matrix material at a temperature of at least above the melting point of the polymeric matrix material, forming the melt into sheet containing molten polymeric matrix material, cooling the sheet containing molten material to form a sheet containing amorphous polymeric matrix material which contains crystallites of the polymeric matrix material, and forming the sheet, while containing the amorphous polymeric matrix material, into a film by stretching the sheet in at least one direction so as to form voids adjacent to at least some of the crystallites and thereby impart opacity to the film. U.S. Pat. No. 6,183,856 is incorporated herein by reference in its entirety.
Regardless of the method of cavitation employed, if the film consists of a single cavitated layer or if the cavitated layer(s) is/are external, then the cavities or voids may be close to the surface of the film or even break through the surface. Such a film may have a rough surface, which may be aesthetically objectionable in certain applications. When void-initiating particles are the means of cavitation, it is typical to solve this problem by a film structure comprising at least 3 layers, where the outermost 2 layers contain no void-initiating particles and are therefore free of voids. This is the approach of the above mentioned U.S. Pat. Nos. 4,632,869, 4,758,462, and 4,652,489. When the additiveless method of the above-mentioned U.S. Pat. No. 6,183,856 is employed to obtain cavitation, at least two different approaches are possible. One is the subject of U.S. Pat. No. 6,194,060, which is summarized in the next paragraph. The other is the subject of the present invention.
U.S. Pat. No. 6,194,060 describes an opaque, oriented polymeric film structure which comprises: (a) a core layer containing a thermoplastic polymeric matrix material which has a first surface and a second surface, the core layer having a plurality of voids, substantially all or all of the voids being free from a void-initiating particle and at least some of the voids being interconnected with an adjacent void in the polymeric matrix material, the number of voids not containing a void-initiating particle being sufficient to impart a significant degree of opacity to the film structure; and (b) at least one outer or skin layer containing a thermoplastic polymeric material and an effective amount of a nucleating agent to suppress cavitation in said at least one outer or skin layer. The film structure has a smooth surface and can be tailored to provide a controlled permeability. U.S. Pat. No. 6,194,060 is incorporated herein by reference in its entirety.
The present invention provides an opaque, oriented polymeric film structure comprising: a core layer comprising a thermoplastic polymeric matrix material wherein the core layer has a first side and a second side, the core layer having a plurality of voids, substantially all or all of the voids being free from a void-initiating particle and at least some of the voids being interconnected with an adjacent void in the polymeric matrix material, the number of voids not containing a void-initiating particle being sufficient to impart a significant degree of opacity to the film structure, wherein the core layer comprises the interior of the film structure; and a first skin layer comprising a thermoplastic polymeric material and an effective amount of a high-melting incompatible additive to suppress cavitation in said first skin layer, wherein the first skin layer is exterior to the first side of the core layer. In another embodiment, the film structure comprises a second skin layer comprising a thermoplastic polymeric material and an effective amount of a high-melting incompatible additive to suppress cavitation in said second skin layer, wherein the second skin layer is exterior to the second side of the core layer.
Advantages of the invention include one or more of the following:
A cavitated thermoplastic film structure having high opacity;
A cavitated thermoplastic film structure having good surface characteristics;
A cavitated thermoplastic film structure which is useful for a variety of applications, including use as packaging films;
A cavitated thermoplastic film structure which is free from, or substantially free from, void-initiating particles;
A cavitated thermoplastic film structure which contains only low amounts of relatively expensive components so as to reduce the material costs in fabricating such films;
A cavitated thermoplastic film structure having a smooth surface;
A cavitated thermoplastic film structure in which the high gas permeability of the film can be controlled and tailored, if desired;
A process for producing a cavitated thermoplastic film structure which has one or more of the aforementioned advantages.
Other embodiments, objects, and advantages of the present invention will be apparent to those skilled in the art upon consideration of the entire present specification, the claims appended hereto, and the accompanying drawings.